Shortly after the ignition of an air-fuel mixture in the cylinder of a two-cycle engine, the moving piston uncovers an exhaust port and a fresh charge of fuel and air is induced into the cylinder. A primary goal of the exhaust system is to remove a maximum of the burned and oxygen-depleted gasses from the cylinder while minimizing the amount of the induced fuel-rich air also removed.
Practical two-cycle engines use resonant exhaust systems adjusted so that a minimum of resistance is offered to the exit of exhaust gasses at one moment while offering resistance to the outward flow of induced fuel-rich gases a moment latter. A conventional resonant exhaust system for a two-cycle engine is a conduit attached to the side of the cylinder. Said conduit flares outwardly from the cylinder's exhaust port to a section of constant cross-section that terminates in a constricting section and a tail-pipe. The outwardly flared section, sometimes called the megaphone, facilites the easy exit of the burned gasses from the cylinder. The constricting section reflects a mechanical wave optimally timed to arrive back at the cylinder in time to tend to impede the exit of fuel-rich air from the cylinder and to increase slightly the pressure of the fuel-rich air in the cylinder above atmospheric pressure. The combination of facilitating removal of the oxygen depleted gasses, retaining the fuel-rich air, and pressurizing of the fuel-rich air results in enhancing the power delivered by the engine.
The delay between the pulse of the exhaust gasses entering the megaphone and the arrival of the reflected wave at the cylinder, is dependent on the propagation velocity (taking into account the temperature and composition of the gasses) and the effective distance between the cylinder and the reflection zone. This distance (and the effective band-width of the active portion of the conduit) is such that the reflected wave is effective over only a certain RPM range. The effective range is usually centered slightly below the RPM that produces a maximum or peak power (the peak RPM) with the result that power out of the engine is only enhanced over a relatively narrow RPM range in the vicinity of peak RPM. If the effective range is centered significantly below the RPM that produces peak power then performance is enhanced in that range of RPM, but performance is adversely affected where hitherto peak power had been attained. In other words, the reasonable design of a conventional resonant exhaust system uses a resonance to improve performance only over a span of RPM near the RPM that produces peak power. A desire to enhance performance at lower RPM (midrange), without compromising performance near peak RPM, has long been felt.
Various attachments or modifications have been made to the above described conventional resonant exhaust system with the goal of improving midrange performance. It has been known for go-cart enthusiasts to form the constricted section and tail pipe, of a conventional resonant exhaust system, in a sliding form (similar to a trombone slide) and to adjust the effective length while the vehicle is moving, and the engine is running, so as to maintain optimum performance for the instant RPM! It is known to use mechanical valves to modify the exhaust system depending on the instant state of the engine. U.S. Pat. No. 4,558,566 discloses the use of a valve "at the inlet of the resonance chamber for opening or closing the inlet in a predetermined operation region of the engine". U.S. Pat. No. 4,554,785 discloses an exhaust apparatus "having a connecting line which connects a sub-chamber to an engine exhaust line and which is provided with an opening and closing value connected to a power source responsive to the number of engine revolutions per minute." Additional U.S. patents that disclose the use of valves in the exhaust system include: U.S. Pat. Nos. 4,545,200, 3,703,937, and 4,570,439. The use of moving parts in or near exhaust gasses inherently presents reliability problems. A need exists in the art for a method of designing modifications to a conventional resonant exhaust system that involve no parts that are moved while the engine is in operation and a method that can be applied to two-cycle engines generally.
Accordingly, an object of the present invention is to provide a device for enhancing midrange performance of an engine using a conventional resonant exhaust system without significant diminution of performance in the vicinity of peak RPM and to teach a method for making such a device.
More particular objects of the present invention are for the device of the present invention not to use any moving parts and to be able to be designed to be effective for essentially any two-cycle engine using a conventional resonant exhaust system.
Further objects of the present invention include applying the method to a plurality of cylinders and teaching alternative forms for the device.